This invention relates to gate valves and particularly to a "soft seal" type gate valve having controlled overtorque stem failure modes and a simplified seal system.
Gate valves of various types have been in use for many years. In general, gate valves are so called "hard seal" valves in which the gate valve is sealed in a metal to metal or hard seal. Such hard seal valves have numerous drawbacks in use. First, in order to make a seal the seal surfaces must match perfectly. This requires very precise machining and fitting of the valve parts which is expensive. Secondly, the seal faces are readily damaged and the seal leaks. Thirdly, the seal surfaces are likely to stick because of corrosion, gum formation or any of a variety of reasons, resulting in the valve becoming inoperative or broken by attempts to release it. More recently, "soft seal" gate valves have been proposed. These valves have elastomer inserts at the sealing surfaces to make and complete the seal under pressure. These valves solve the problems discussed above in connection with hard seal gates valves, but have problems of their own. The known soft seal valves most often use double matching piece gate construction where the gate pieces are mechanically fastened around the seal member. The double piece construction is expensive to form. Moreover, it has been found that fluid leakage results around the fasteners holding the pieces. Leakage also results when the pieces are slightly misaligned due to machining and assembly errors. It is also possible for the fasteners to fail when the gate having misaligned pieces is urged into engagement with the valve body in the fluid passage. Other soft seal valves propose using a single piece gate with the seal molded onto the gate body. The single piece gate construction requires special equipment and elaborate processes to produce and as a result is expensive to manufacture.
My invention also provides for overtorque failure of the stem. This feature is significant in that it allows for continued use of the gate valve after an initial stem failure and also avoids internal damage to the valve gate and other internal portions of the valve. My overtorque feature is best understood by considering the problems inherent in existing gate valves from overtoruqe stem failure. One of the most common causes of valve failure is overtorquing the valve stem. When such a failure results the gate valves are most often replaced or repaired at great expense compounded when the gate valve is in an underground setting and must be dug out of the ground upon failure. If the internal damage is not excessive the valve may still need to be removed and the damaged parts replaced. In either case the gate valve must be replaced or repaired since it is no longer functional. A most serious problem arises when an overtorque failure occurs to a gate valve that is in a hazardous service application such as in a natural gas pipeline and there is a fire downstream of the valve. Often in this type of emergency the valve may not be capable of being fully closed when first operated and as a result gas will continue to flow into the fire zone. Due to the critical nature of the emergency, additional torque is normally applied to the valve stem in an attempt to effect a tight closure. Although the valve may be finally closed, time may be lost and the additional torque applied to the valve stem may result in a stem failure or damage to internal parts of the valve locking the gate in a closed position and making further operation of the valve impossible. The fire may be contained, however, the inoperable valve may create another emergency as critical as the first. This second emergency may occur during inclement weather when an urgen need exists to quickly reopen the valve in order to reinstate gas service to high risk users such as hospitals, schools and the elderly. In this case the inoperable valve may greatly increase the risk to these users. It is therefore absolutely necessary that a valve function to close as well as to open, esepcially in emergency situations as described. In addition, because time-is-of-the-essence in many emergencies, it is just as critical that the operation of a valve be accomplished in the minimum of time. Gate valves currently in use do not provide this margin of operational dependability.
This invention overcomes the aforementioned problems related to hard seal gate valves as well as those problems of known soft seal gate valves. My invention also overcomes the overtorque failure problems of all known existing gate valves and in doing so provides the extra margin of operational dependability required in emergency situations. My gate valve is simply constructed with a valve stem having two wrench head positions with respective zones of weakness designed to fail in torsion at predetermined and progressively greater magnitudes of torque. Under normal service conditions the stem strength in both wrench head positions is more than sufficient to operate the valve. Should the valve stem be overtorqued the first or outermost wrench head will fail at the first zone of weakness at a predetermined low magnitude of torque. This first controlled failure will result in the complete detachment of the wrench nut and first wrench head. The magnitude of overtorque required to fail the first zone of weakness is kept so low that no damage is caused to any other elements of the valve. The valve remains fully operational as the second wrench head position becomes accessible and may be easily grasped by the original wrench nut or by a standard socket drive member in order to continue rotating the stem to operate the valve gate. In the second wrench head position a predetermined overtorque greater in magnitude than the first but less than that of the main body of the valve stem would be necessary to cause failure in the second zone of weakness. My unique valve gate is formed of a single piece body having a shape suitable for receiving a seal member which is easily installed and removable from the valve gate. My overtorque protection stem and valve gate seal combination may be fabricated at costs allowing the gate valves to be sold at prices acceptable to customers.